Ultrasonic distance measurement has applications in agriculture in measuring and controlling depth of cultivation, depth of planting, the relative position of a header comb in harvesting, and other operations where non-contact distance measurement is required. Various proposals have been made for ultrasonic distance measurement including applications in the agricultural environment, for example as described in the present assignees Austrialian petty patent No. 545,608.
Antennas for various radiation systems are known including parabolic reflectors. With a transducer located at the focus of a parabolic reflector, a parallel beam of circular cross-sectional shape is produced. However, there are practical difficulties in mounting a transducer in such a position especially when apparatus suitable for ultrasonic use in agricultural applications is contemplated. Ruggedness and high resistance to vibration, shock and dust are required for agricultural purposes. Another problem of using a simple parabolic reflector arrangement is that the transducer mounting would be in the path of the beam and this would not only reduce the transmitted and received signal but also, very importantly, cause diffraction and subsequent widening of the beam.
Publications in the antenna field include U.S. Pat. No. 4,208,661 (Vokurka) and U.S. Pat. No. 3,792,480 (Graham). Vokurka concerns the use of two parabolic cylindrical surfaces co-operating with a point source of radiation. Graham discloses a system suitable for microwaves i.e. electromagnetic radiation and teaches the use of first and second reflectors respectively convex and concave and being, for example, part hyperbolic or part parabolic. The system provides an accurate parallel beam.
Neither Vokurka nor Graham address the question of ultrasound systems, (in which transducers are not of a point source type) and produce sound waves which compared with electromagnetic waves are (a) longitudinal waves and (b) of very much greater wavelength.
The present invention is directed towards a product using ultrasound and suitable for use in agricultural applications and this imposes considerable problems. The ground surface in agricultural applications varies considerably. Whereas ultrasonic height sensing from a smooth hard surface such as a roadway does not present problems, there is difficulty in producing reliable readings over a variety of different ground surfaces. Some surfaces will have been worked and therefore have hard lumps and stones on the surface as well as having a generally powdery characteristic resulting in considerable scatter of radiation and attenuation of the ultrasound signal rather than good reflection.
Furthermore, economic factors require that the sensor equipment be not only extremely rugged and resistant to vibration and dust but also must have a relatively low capital cost. Existing commercial ultrasonic transducers are acceptable in cost but have fairly wide-angle beams. A typical ultrasonic transducer has a signal amplitude of 70 percent of the axial value at a position 20.degree. off its directional axis and even at 70.degree. off the directional axis the signal amplitude can be 15 percent of the axial value. When these transducers are used to measure a target surface of varying reflectivity, for example the ground surface, it is common to obtain confusing echoes reflected from machine parts and the ground surface in a direction well off the directional axis of the transducer. In these circumstances it is difficult, and perhaps impossible, to obtain satisfactory settings of receiver gain and measurement thresholds to suit all operating conditions.
Accordingly it would be desirable to provide a well defined and controlled ultrasonic signal beam with a comparetively narrow form suitable for projecting a distance of the order of 650 mm from the machine mounting to the ground surface.